Social insects, examples of which are ants, bees, wasps and termites, communicate using pheromones which are chemical messengers acting between members of the same species. Threat or danger to the colony is communicated by volatile alarm pheromones.
Research carried out on social insects has shown that alarm pheromones are generally a complex mixture of the chemical compounds. Such a mixture will usually alert, alarm and attract nestmates to the release site, where they attack any intruder. In some species, there is evidence that different compounds within a mixture affect selected aspects of behaviour, such as orientation towards the source or biting.
In some areas the leaf-cutting ant is a particular pest. Control of these pests is carried out with the use of a bait comprised of a carrier (often dried citrus pulp) impregnated with a toxicant, for example Mirex (trade mark), enclosed in a plastic sachet for protection against rain and decomposition. Mirex (TM) is an ant bait with Dodecachlorooctahydro-1,3,4 metheno-2H-cyclobuta (cd) pentalene as its active ingredient. The insecticide is formulated in granules of material of vegetable origin such as dried citrus pulp. The sachets are distributed on the ground in areas where the ants are active. When cut open by the ants the contents are taken to the nest as a food source and the toxicant is spread around the colony, usually causing its death. Such baits whether initially protected in plastic envelopes or not must be discovered and exploited very quickly by the ants or they will decompose rapidly in a humid tropical environment. Therefore, anything which increases the attractiveness of the bait and improve pick-up rate would be extremely useful.
Other social insects to which are particular pests include members of the Atta genus, such as Atta sexdens, Atta laevigata and Atta bisphaerica, which are also leaf cutting ants, and members of the Solenopsis genus, fire ants.
The invention provides an insecticidal system for social insects comprising a means for killing the insects together with synthetic equivalent to at least one component of the alarm pheromone of the insect.
In a preferred embodiment the insecticidal system is a composition comprising a synthetic equivalent to at least one component of the alarm pheromone of the insect, together with an insecticide for the insect.
Once the insects have been attracted to the bait, they may be killed either in situ by, for example, being killed in a trap and retained therein or exposed to insecticides therein, or by being provided with a slow-acting toxicant which they take to their nest, where it kills large numbers of insects.
Leaf cutting ants live in symbiosis with a fungus which breaks down plant material into a form assimilable by the ants. Incorporation of a fungicide, such as pentachloronitrobenzene or Benomyl, for this fungus in a system according to the invention will result in the death of the fungus and so the nest.
The invention also provides a method of controlling social insects comprising using a synthetic equivalents to at least one component of the alarm pheromone of the insect to attract the insect to an insecticide for the insect or to another means of killing the insect. The composition is placed suitably close to the nests of the insects.
Recent research by the applicant on the neotropical leaf-cutting ant Acromyrmex octospinosus has shown that t components of the alarm pheromone of this ant, namely 3-octanol and 3-octanone, attract nestmates and induce the laying of trails around the source which ensure that nestmates are then attracted towards the source for some time and attack the source on arrival. It appears that 3-octanone mainly causes recruitment of ants from the peripheral area, while 3-octanol mainly controls attraction to the source, the laying of pheromone trails radiating out from the source, and biting.
The natural alarm pheromone of the ants is unsuitable for use as a bait for subsequent incorporation in the nest. The concept of alarm pheromones in general implies that they are totally inappropriate substances for use in food baits as such baits would then be treated as aliens or enemies and attacked and removed.
It has now been found in laboratory trials that if 3-octanol and 3-octanone are synthesised, the synthetic chemicals will prolong the sequence of alerting, marking with trail substances around the source, biting and recruitment of nestmates from the peripheral area, especially when both chemicals are used together. This is significantly different from the behaviour induced by release of the natural pheromone from crushed workers, in which the above sequence is very shortlived, terminating in arrestance of nestmates at the source for long periods without attack and with subsequent removal of the body to rubbish dumps outside the nest. It is expected that this surprising effect will be shown by synthetic components of pheromones of other insects, especially social insects.
The sequence of behaviour induced in the ant by the two synthetic compounds (which do not represent the totality of compounds in the natural secretion) is a magnified and prolonged section of the natural sequence of behaviour induced by the naturally produced pheromone. The synthetic pheromone compounds may therefore be used in food baits in contrast to what would be expected.
Similar behaviour can be induced in other insect species, such as Atta, including A. sexdens, A. laevigata and A. bisphaerica, and Solenopsis (fire ants) by use synthetic alarm pheromone components. Synthetic alarm pheromone components which are useful also include nonanol, decanal dodecanal, 2-phenylethanol, citral, farnesol, 6-methyl-5-hepten-2-one, 4-methyl-3-heptanone, decanoic acid, geraniol, tetradecanal or xcex2-pinene.